This invention relates generally to amplifiers and more particularity to operational amplifiers.
As it is known in the art, a operational amplifier is commonly used to provide an amplified difference output voltage provided in response to a difference voltage between two input signals. Generally the operational amplifier is configured to receive two input signals at respectively its inverting and non-inverting inputs to produce in response the amplified difference output voltage.
Several characteristics of an ideal operational amplifier are that the amplifier has a very high voltage gain, a low output impedance, high input impedance, and high bandwidth. The exact specifications for each of these parameters will vary for real devices, but in general for any real device the specification of any of them can be generally characterized as mentioned above.
As is also known, operational amplifiers also have a characteristic called D.C. offset voltage. The D.C. offset voltage of an operational amplifier can be viewed as that voltage difference required at the inputs of the amplifier to provide a zero output voltage relative to a reference voltage. For ideal amplifiers the D.C. offset voltage is zero whereas for actual amplifiers it is generally several millivolts or more. This D.C. offset voltage is thus an error type voltage and in general is undesirable.
Techniques are known to compensate for this D.C. offset voltage. One technique involves the use of trimming an electrical characteristic of a component such as a resistor to reduce the D.C. offset. This technique while feasible with hybrid circuits is generally not feasible with monolithic integrated circuits. Another technique is to cascade several amplifiers together to reduce offset. This technique has several drawbacks including high cost and reduced bandwidth. In an operational amplifier, the design thereof can be modified to provide a higher voltage gain to swamp out the effects of the D.C. offset voltage. The chief problem is that the bandwidth of the amplifier is reduced and moreover such a circuit is potentially unstable and thus measures must be taken to make it stable.